A winter tyre is supposed to ensure good traction and braking characteristics when used on snow (and possibly ice). However, a winter tyre also needs to provide good traction and braking characteristics when used on wet roads. Moreover, a winter tyre also needs to provide good driving performance on dry roads and low noise.
Winter or snow tyres have been designed with treads having traction blocks widely spaced by large and deep grooves that allow a good traction and brake on snow. The presence of large and deep traction grooves increases the void-to-rubber ratio of the tread, i.e., the amount of open space compared to the amount of rubber in the tread. A low void-to-rubber ratio means more rubber in contact with the road and vice versa. The high void-to-rubber ratio tread pattern is excellent in deep snow because snow is trapped in the void spaces and increases the grip. However, these tread patterns reduce the stiffness of the tread and generate substantial noise and block mobility when used on wet or dry roads. Moreover, the tread wear rate is rapid and therefore these tyres are considered for use only during the snowy winter months.
The use of treads having a high density of sipes made on the blocks has provided a good improvement to both snow and ice traction performance. Such heavily bladed tyres can exhibit good winter ice traction performance by providing an increased number of tread edges to provide forward traction.
Furthermore, the performance of a tread on wet roads is increased by the presence of grooves able to drain water from the contact surface of the tread with the road. This is accomplished in the art by longitudinal or transverse grooves. The wider the groove, the higher the amount of drained water. However, broad longitudinal grooves reduce the traction on snow and broad transverse grooves reduce the stiffness of the tread, again increasing the noise, block mobility and wear when driving on dry road. WO02/068221, WO02/068222, U.S. Pat. No. 5,435,366, EP 139,606, U.S. Pat. No. 5,088,536, WO08/074353 disclose exemplary known tread patterns which provide snow or all season performance.